1. Field of the Invention
The present invention relates to a thin film transistor substrate used in a display device or the like.
2. Description of the Background Art
A TFT active matrix substrate (thin film transistor substrate; hereinafter referred to as a “TFT substrate”) using a thin film transistor (hereinafter referred to as a “TFT”) as a switching element is used in an electrooptical device such as a display device (liquid crystal display) using liquid crystal. A semiconductor device such as a TFT has features of reduced power consumption and thin size, and it has actively been applied to a flat panel display that replaces a CRT (Cathode Ray Tube) by utilizing these features.
An electrooptical element for a liquid crystal display (LCD) includes a simple matrix LCD and a TFT-LCD using a TFT as a switching element. The TFT-LCD is particularly more excellent than the CRT or the simple matrix LCD in portability and display quality, so that it has widely been put into practical use such as a notebook personal computer.
In general, a TFT-LCD includes a liquid crystal display panel having a liquid crystal layer held therein between a TFT substrate having a plurality of TFTs arranged in an array and a counter substrate having a color filter and the like. A polarization plate is provided on each of a front surface and a back surface of the liquid crystal display panel, and a backlight is further provided on one of these surfaces. This configuration can realize a satisfactory color display.
An LCD with an IPS (In Plane Switching) system that is a liquid crystal driving system using a transverse electric field and attained by improving a viewing angle of a conventional TFT-LCD has widely been used for a display device and the like by utilizing the feature of wide viewing angle. However, it also has problems of low opening ratio and low transmittance on a pixel display portion, and it is difficult to attain a bright display characteristic. The main reason for this is that an electric field for driving liquid crystal is not effectively applied above an interdigital pixel electrode used for the IPS-LCD, and hence, some liquid crystals on the pixel electrode do not operate. In order to solve this problem, an LCD with an FFS (Fringe Field Switching) system described in Japanese Patent Application Laid-Open No. 2001-56474 has been proposed.
A TFT substrate in a general FFS-LCD described in Japanese Patent Application Laid-Open No. 2001-56474 is formed through at least six photolithography processes including (1) a process of forming a gate electrode, (2) a process of forming a pixel electrode, (3) a process of forming a gate insulating film and a semiconductor film, (4) a process of forming source and drain electrodes, (5) a process of forming contact holes on a protection insulating film, and (6) a process of forming a common electrode.
Amorphous silicon (Si) is conventionally used as a semiconductor film serving as an active layer (channel layer) in a switching element in a TFT substrate for a liquid crystal display. Recently, development of a TFT using an oxide semiconductor for an active layer has been actively made. An oxide semiconductor has mobility higher than that of a conventional amorphous silicon. As the oxide semiconductor, zinc oxide (ZnO) materials or amorphous InGaZnO materials formed by adding gallium oxide (Ga2O3) and indium oxide (In2O3) to zinc oxide are mainly used. This technique is described in Japanese Patent Application Laid-Open No. 2005-77822, Japanese Patent Application Laid-Open No. 2007-281409, and Kenji Nomura et al., “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”, Nature 2004, vol. 432, pages 488 to 492.
An oxide semiconductor material can be etched by a weak acid solution such as oxalic acid or carboxylic acid, like an oxide conductor such as amorphous ITO (indium oxide (In2O3)+tin oxide (SnO2)) that is a transparent conductor or amorphous InZnO (indium oxide (In2O3)+zinc oxide (ZnO)), so that it has an advantage of easy pattern formation.
However, the oxide semiconductor material is easy to be dissolved even in an acid solution used for an etching process of a metal film (Cr, Ti, Mo, Ta, Al, Cu, and an alloy of these metals) generally used for a source electrode or a drain electrode in a TFT. Accordingly, when a TFT having a structure in which a source electrode and a drain electrode are arranged on an upper layer of an oxide semiconductor is manufactured as illustrated in FIG. 11 in Japanese Patent Application Laid-Open No. 2007-281409, a selective etching in which only a metal film of the source electrode and the drain electrode is etched and the oxide semiconductor is not etched and left is difficult.
In order to solve this problem, it is considered that a TFT structure including a semiconductor film as an active layer formed on a source electrode and a drain electrode is employed as illustrated in FIG. 1 in Japanese Patent Application Laid-Open No. 2003-92410 and FIG. 1A in Japanese Patent Application Laid-Open No. 2006-5329, for example. In this TFT structure, it is only necessary that, after the gate electrode, the source electrode, and the drain electrode are formed by processing the metal film, the semiconductor film made of an oxide semiconductor is formed. Therefore, there is no chance that the semiconductor film is dissolved in an acid solution upon etching the metal film. In addition, since a weak acid solution such as oxalic acid or carboxylic acid used upon etching the oxide semiconductor does not etch a normal metal, the source electrode and the drain electrode are not etched during the formation of the semiconductor film. Accordingly, the selective etching of the semiconductor film made of an oxide semiconductor and the metal film can be carried out, whereby a high-performance TFT substrate having high mobility can be manufactured.
There is no problem in the case described in Japanese Patent Application Laid-Open No. 2001-56474 in which the transparent conductive film pattern becomes the uppermost layer. However, as described in Japanese Patent Application Laid-Open No. 08-6059 (1996), when an insulating film (hereinafter referred to as “upper insulating film”) such as a protection film or an interlayer insulating film is formed on a transparent conductive film pattern, stress of the transparent conductive film and stress of the upper insulating film is unbalanced, so that a phenomenon called “film floating” or “film stripping” (hereinafter collectively referred to as “film floating”) in which the upper insulating film is peeled on the edge of the transparent conductive film pattern might occur. The film floating prominently occurs on a region where the pattern density is relatively coarse, such as on a frame region outside a display region, e.g., on an external connection terminal portion or a wiring conversion portion. The occurrence of the film floating deteriorates the function of the upper insulating film as the protection film to cause corrosion, or deteriorates the function of the interlayer insulating film to cause breakdown. Therefore, when the film floating occurs, yield of products and reliability is deteriorated. In addition, the peeled upper insulating film scatters in a manufacturing device to generate dust, which adversely affects other products manufactured by the same manufacturing device. This also deteriorates yield of products and reliability.
On the other hand, when a film-forming condition that makes it difficult to cause the film floating due to the balanced stress is employed upon forming the upper insulating film, problems arise including a problem of reduction in transmittance of the transparent conductive film pattern, and a problem of occurrence of connection failure between a line and a pixel electrode due to a formation of a wedge-like gap (hereinafter merely referred to as “wedge”) on an interface of a gate insulating film and an interlayer insulating film in a contact hole connecting the line and the pixel electrode. Particularly, in a liquid crystal display highly demanded to have high brightness (high open ratio and high transmittance) and wide viewing angle, enhancement in transmittance of a transparent conductive film and an adoption of an FFS system are inevitable, which increases the case in which such a film-forming condition as to be liable to generate the film floating in the upper insulating film has to be employed.
In addition, in the FFS system, it is essential that a pixel electrode and a common electrode both made of a transparent conductive film are arranged opposite to each other via an interlayer insulating film, so that the interlayer insulating film (upper insulating film) is unavoidably arranged on at least one of the transparent electrodes. Therefore, a countermeasure for the film-floating problem has to be devised.
When a film made of an oxide semiconductor such as ZnO or InGaZnO is directly formed on a metal film (Cr, Ti, Cu, Mo, Ta, Al, or an ally of these metals) serving as a source electrode or a drain electrode of a TFT by using a sputtering method or a vacuum deposition method, an oxide layer of the metal film is formed on the interface between both films due to an interface reaction, whereby the electric resistance (interface resistance) increases.
According to experiments conducted by the present inventors, when a metal film made of Al is formed on an oxide semiconductor film made of InGaZnO (ratio of number of atoms: In:Ga:Zn:O=1:1:1:4), for example, an interface resistance value per an area of 50 μm×50 μm is 200 kΩ, while an interface resistance value per the same area is 100 MΩ or more, when the formation order of the Al film and the InGaZnO film is reversed. As for other metals (Cr, Ti, Cu, Mo, Ta), the interface resistance value increases in single or more digits, when the formation order of the metal film and the InGaZnO film is reversed, as in the case of Al. The same applies to the case where an alloy film having these metal films as a main component (the component having the highest ratio of number of contained atoms) is used.
On the other hand, a reduction reaction with the metal film occurs on the oxide semiconductor film, so that an oxide semiconductor film with insufficient oxygen is generated on the channel surface near the interface. In the oxide semiconductor film with insufficient oxygen, the carrier density increases to reduce resistance, which entails a problem of an increase in off current of the TFT. In the TFT structure in which the semiconductor film made of the oxide semiconductor is formed on the source electrode and the drain electrode, which are made of the metal film, the interface reaction layer increases. Consequently, the deterioration in on/off performance of the TFT and the reduction in mobility are generated, which entails a problem of deterioration in TFT characteristic.